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RocketMotorTwo is a hybrid rocket engine being developed for the Scaled Composites SpaceShipTwo suborbital spaceplane. The engine is being designed and built by Sierra Nevada Corporation (SNC),[1] and was first ignited in flight during a SpaceShipTwo test launch in April 2013.[2][3]


RocketMotorTwo is a throttleable hybrid rocket engine fueled with hydroxyl-terminated polybutadiene (HTPB) and nitrous oxide – sometimes referred to as an N2O/HTPB engine[4][5] – providing 60,000 pounds-force (270 kN) of thrust.[6]


As of March 2013, Sierra Nevada has performed over 300 hybrid rocket test firings. The company also developed the rocket engine for the first private spacecraft to reach space, SpaceShipOne, which won the Ansari X-Prize in 2004. Sierra Nevada is furthermore developing a similar hybrid engine, the Dream Chaser rocket engine.[7]

Development history[edit]

Hot-fire ground tests[edit]

Scaled Composites performed a series of subscale rocket hot-firings between June 2005 and April 2009, before choosing a full-scale rocket motor design.[8] By December 2012, 21 full-scale hot-fire ground tests had been carried out on RocketMotorTwo.[8][9][10] On 20 June 2012, the first hot-fire test under the control of SpaceShipTwo's proprietary Rocket Motor Controller (RMC) was successfully conducted.[8]

Test program accident[edit]

On 26 July 2007, during the early rocket testing phase, an explosion occurred during a propellant flow test at the Mojave Air and Space Port. The test included filling the oxidizer tank with 4,500 kg (10,000 pounds) of nitrous oxide, followed by a 15-second cold flow injector test. Although the tests did not ignite the gas, three employees were killed and three injured, two critically and one seriously, by flying shrapnel.[11]

Commencement of flight testing[edit]

In July 2012, Virgin Galactic, the owner of VSS Enterprise – the first SpaceShipTwo spaceplane built – announced that RocketMotorTwo was fully qualified for powered flight, and that the ground test program was substantially complete.[12] SpaceShipTwo conducted its first gliding test flight with the engine fully installed in December 2012.[13] Additional ground rocket tests continued into March 2013 as the company prepared for powered test flights.[14]

VSS Enterprise conducted its first powered flight on 29 April 2013,[2] marking the first flight test of RocketMotorTwo. The test was a 16-second burn as planned, and was ignited at an altitude of 47,000 feet (14,000 m), shortly after Enterprise was released from its WhiteKnightTwo carrier aircraft and the pilots cross-checked data and verified stable control. The RM2 control system opened the main oxidizer valve and fired the igniters inside the fuel case. The burn was completed at an altitude of 55,000 feet (17,000 m), by which point Enterprise was supersonic, achieving Mach 1.2 (790 mph; 1,300 km/h). SNC stated after the test that "The rocket motor ignition went as planned, with the expected burn duration, [and] good engine performance."[3]

2014 Change of fuel[edit]

In May 2014, Virgin Galactic announced a change to the fuel to be used in the SpaceShipTwo rocket engine. Rather than the rubber-based HTPB—HTPB engines had experienced serious engine stability issues on firings longer than approximately 20 seconds—the engine will now use a type of plastic called thermoplastic polyamide as the solid fuel. The plastic fuel is projected to have better performance (by several unspecified measures) and will allow SpaceShipTwo to make flights to a higher altitude.[15][16][17]

As of May 2014, the new engine formulation has already completed full-duration burns of over 60 seconds in ground tests on an engine test stand.[16] However, four additional ground tests of the polyamide-fueled engine are anticipated before the SpaceShipTwo flight test could resume with the new-fuel rocket motor.[17]


The hybrid rocket motor and its oxidizer valve system are produced in SNC’s manufacturing facility in Poway, California in conjunction with Scaled Composites. In 2013, the Poway facility was reported to be "currently producing motors for both SpaceShipTwo and SNC’s own Dream Chaser orbital crew vehicle".[3]

See also[edit]


  1. ^ Leonard David. "Virgin Galactic Pushes Private Spaceship Envelope in Test Flights". 10 June 2011. Retrieved 19 May 2013.
  2. ^ a b "Sir Richard Branson's Virgin Galactic spaceship ignites engine in flight". BBC. 29 April 2013. Retrieved 29 April 2013. 
  3. ^ a b c Messier, Doug (29 April 2013). "SNC: Yeah, We Lit that Candle and it Worked, Baby!". Parabolic Arc. Retrieved 19 May 2013. 
  4. ^ "Propulsion Systems: multiple-burn, green and low-cost" (PDF). Sierra Nevada. Retrieved 8 March 2013. 
  5. ^ "Safe Hybrid Rocket". Overview – Safety. Virgin Galactic. 2013. Retrieved 8 March 2013. 
  6. ^ "SpaceShipTwo performs first Rocket-Powered Flight". 29 April 2013. Retrieved 19 May 2013. 
  7. ^ "Propulsion Systems". Sierra Nevada Corporation. Retrieved 2012-12-09. 
  8. ^ a b c "RocketMotorTwo Hot-Fire Test Summaries". 9 August 2012. Retrieved 10 December 2012. 
  9. ^ Lindsey, Clark (20 December 2012). "SpaceShipTwo rocket motor tests in Nov. and Dec.". NewSpace Watch. Retrieved 22 December 2012. (subscription required (help)). 
  10. ^ "Virgin Galactic successfully completes SpaceShipTwo glide flight test and rocket motor firing on same day". 28 June 2012. Retrieved 19 May 2013.
  11. ^ Abdollah, Tami and Silverstein, Stuart (27 July 2007). "Test Site Explosion Kills Three". Los Angeles Times. Retrieved 27 July 2007.
  12. ^ Ruddick, Graham (11 July 2012). "Farnborough Airshow: Branson's Virgin Galactic 'spaceship' in UK debut". The Telegraph. Retrieved 13 July 2012. 
  13. ^ "SpaceShipTwo straps on its engine". NBC. 20 December 2012. Retrieved 20 December 2012. 
  14. ^ Richard Branson (5 March 2013). "This isn't sci-fi". Retrieved 5 March 2013. 
  15. ^ Foust, Jeff (2014-05-24). "Virgin Galactic changes fuels as it prepares for its next round of test flights". NewSpace Journal. Retrieved 2014-05-25. 
  16. ^ a b Boyle, Alan (2014-05-23). "Virgin Galactic Makes a Switch in SpaceShipTwo's Rocket Motor". NBC News. Retrieved 2014-05-24. 
  17. ^ a b "New Fuel to Boost SpaceShip Two". Aviation Week. 2014-05-24. Retrieved 2014-05-27. 

External links[edit]